1. Field of the Invention
This invention relates to a novel method of tracking servo for an optical pickup device, and more specifically, to a technique which permits highly accurate tracking servo particularly in recording in an optical pickup device for use in a disk drive of a recordable optical disc as well.
2. Description of the Related Art
DPP (Differential Push Pull) method is well known as a method of tracking servo in a disk drive of an optical recording medium such as CD (Compact Disc), for instance.
The DPP method is a method of generating a tracking error signal through an operation of signals respectively outputted from photo detectors on the basis of a main beam MB and two side beams SB, SB.
Specifically, the DPP is by forming three spots respectively based on three beams, i.e., 0th-order diffraction light (a main beam) and xc2x11-st order diffraction light (side beams) on an optical disc with a diffraction means (a grating) arranged in a going path of a beam emitted from a laser light source, and then receiving returned light of the above three beams with photo detectors a, b, b respectively for applying a main spot MS based on the main beam to writing or reading of signals, while applying side spots SS, SS based on the side beams to tracking error detection.
As shown in FIG. 1, for instance, the main photo detector xe2x80x9caxe2x80x9d for detecting the main spot MS is split into four sections crosswise, while each of the side photo detectors xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d for detecting the side spots SS, SS is split into two sections left and right. Incidentally, output signals from the respective split elements are denoted as A, B, C, D, E, F, G and H. Then, a tracking error signal is generated on the basis of operational output among the output signals from these photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d.
That is, a DPP (Differential Push Pull) signal is obtained according to the following operation expression on the basis of an MPP (Main Push Pull) signal generated from the output signal of the main photo detector xe2x80x9caxe2x80x9d and SPP1 and SPP2 (Side Push Pull) signals generated from the output signals of the side photo detectors xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d.
MPP=(B+C)xe2x88x92(A+D)
SPP1=Exe2x88x92F
SPP2=Gxe2x88x92H
DPP=MPPxe2x88x92Kxc2x7(SPP1+SPP2)
∴DPP=((B+C)xe2x88x92(A+D))xe2x88x92Kxc2x7((Exe2x88x92F)+(Gxe2x88x92H))
K: Coefficient
FIG. 1 shows schematically the relation between the photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d and the spots MS, SS, SS when optical components or the like are free from variations, and FIG. 2 shows waveforms of the SPP1 signal, the SPP2 signal, the MPP signal and the DPP signal in FIG. 1.
As is apparent from FIG. 2, the SPP1 signal and the SPP2 signal are outputted in the same phase, the MPP signal is outputted in the reverse phase to the SPP1 signal and the SPP2 signal, and the DPP signal is outputted in the same phase as the MPP signal.
FIG. 3 schematically shows the positional relation between the photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d and the spots MS, SS, SS when spaces among the spots MS, SS, SS are different due to variations or the like in optical components (the widened spaces among the spots are shown in FIG. 3). FIG. 4 shows the waveforms of the SPP1 signal, the SPP2 signal, the MPP signal and the DPP signal in FIG. 3. Incidentally, the waveform shown by a solid line in FIG. 4 is of each PP (Push Pull) signal in signal reading, while the waveform shown by a broken line is of the SPP signal and the DPP signal in signal writing which will be described later.
In signal reading, although positive DC offset and negative DC offset are respectively produced in the SPP1 signal and the SPP2 signal, both the amounts of DC offset are of the same absolute value. Thus, these amounts of DC offset may be canceled according to the above operation expression to permit the DPP signal to be outputted in a DC offset free state, resulting in maintenance of an on-track state (Refer to a solid line in FIG. 4).
FIG. 5 schematically shows the positional relation between the photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d and the spots MS, SS, SS when the spots MS, SS, SS are one-sidedly (rightward in FIG. 5) shifted on the photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d due to the positional difference of the objective and the photo detector or the like. FIG. 6 shows the waveform of the SPP1 signal, the SPP2 signal, the MPP signal and the DPP signal in FIG. 5. Incidentally, the waveform shown by a solid line in FIG. 6 is of each PP (Push Pull) signal in signal reading, while the waveform shown by a broken line is of each PP (Push Pull) signal in signal writing which will be described later.
In signal reading, although DC offset is produced in any of the SPP1 signal, the SPP2 signal and the MPP signal, these amounts of DC offset may be canceled according to the above operation expression to permit the DPP signal to be outputted in the DC offset free state, resulting in maintenance of the on-track state
As described the above, the DPP method is considered to be an extremely effective method of tracking servo for an optical pickup of a signal reading system, for the reason that the amount of DC offset of the tracking error signal (the DPP signal) may be canceled according to the above operation expression, even in any of the case where the spaces among the spots MS, SS, SS are different due to the variations or the like in optical components as shown in FIG. 3, and the case where the spots MS, SS, SS are one-sidedly shifted on the photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d due to the positional difference of the objective lens and the photo detector or the like as shown in FIG. 5.
However, it is to be understood that only the case of reading the signals of the optical disc as described above permits the amounts of offset produced to be canceled, when the spaces among the spots MS, SS, SS are different due to the variations or the like in optical components as shown in FIG. 3 or when the spots MS, SS, SS are one-sided on the photo detectors a, b, b due to the positional difference of the objective and the photo detector or the like as shown in FIG. 5.
FIG. 7 schematically shows the positional relation between the main beam MB and the side beams SB1, SB2 and recording tracks T in writing the signals onto an optical disc C.
In writing signals onto the optical disc C, one side beam SB1 (the side beam preceding the main beam MB) is reflected from a disc surface placed in a non-written state that pits xe2x80x9cdxe2x80x9d, xe2x80x9cdxe2x80x9d, are not formed yet at all, while the other side beam SB2 (the side beam following the main beam MB) is reflected from the disc surface between tracks T, T placed in the written state that the pits xe2x80x9cdxe2x80x9d, xe2x80x9cdxe2x80x9d, . . . are already formed. Thus, there is a problem in that a method similar to that of canceling the offset produced in reading the signals is not good enough to cancel the offset produced in writing the signals onto the optical disc.
That is, in writing the signals when the spaces among the spots MS, SS, SS are different due to the variations or the like in optical components to produce the offset, a disk drive of an optical recording medium, for example, an optical disc such as CD-R (Compact Disc-Recordable) and CD-RW (Compact Disc-Rewritable) or like optical disc causes the amount of DC offset 1 of the SPP1 signal to be different from the amount of DC offset 2 of the SPP2 signal, as shown by the broken line in the waveform diagram of FIG. 4. Specifically, the SPP2 signal produces the amount of offset 2 less in absolute value than the amount of offset 1 produced in the SPP1 signal, and as a result, the amount of DC offset 3, which may not be canceled by the above operation expression, is produced in the DPP signal provided as a final output signal.
On the other hand, in writing the signals when the spots MS, SS, SS are one-sidedly shifted on the photo detectors xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9cbxe2x80x9d due Lo the positional difference of the objective lens and the photo detector or the like to produce the offset, the disk drive of the optical disc such as CD-R and CD-RW causes the amount of DC offset 1xe2x80x2 of the SPP1 signal to be different from the amount of DC offset 2xe2x80x2 of the SPP2 signal as shown by the broken line in the waveform diagram of FIG. 6. Specifically, the SPP2 signal produces the amount of offset 2xe2x80x2 less than the amount of offset 1xe2x80x2 produced in the SPP1 signal to lower the DC level, and as a result, the amount of DC offset 3xe2x80x2, which may not be canceled by the above operation expression, is produced in the DPP signal provided as the final output signal.
As described above, the disk drive in writing raises the problems in that the application of tracking servo produces a difference by a DC offset portion of the DPP signal to bring detracking (out-of-track), resulting in deterioration of writing performance.
In order to bring the above amounts of offset within the allowable limits in the optical pickup device, there is a need for higher accuracy of optical components themselves and the photo detector itself, together with higher accuracy of installation of these components, resulting in an increase in device cost. Further, the narrower allowable limits are required for the above amounts of DC offset Lo meet a demand for higher recording density, and in consequence, the optical pickup device presents a problem in difficulty in meeting a demand for higher recording density.
It is an object of the present invention to permit highly accurate tracking servo particularly in writing, simultaneously with attainment of higher recording density.
To achieve the above object, a method of tracking servo for an optical pickup device according to the present invention comprises the steps of generating each individual push pull signal with a photo detector composed of a main photo detector having four light receiving sections crosswise and two side photo detectors respectively having two light receiving sections left and right, and then generating a tracking error signal on the basis of operational output of the push pull signals resulting from canceling the amounts of DC offset produced in the push pull signals in the respective photo detectors for each push pull signal.
Accordingly, the method of tracking servo for the optical pickup device according to the present invention permits the amounts of DC offset produced in the push pull signals in the respective photo detectors to be canceled for each push pull signal. Thus, even if the SPP1 signal and the SPP2 signal are different in amount of DC offset, the DPP signal obtained finally from the above signals may be prevented from producing the offset, permitting highly accurate tracking servo simultaneously with attainment of higher recording density.
As has been described in the foregoing, in the method of tracking servo for the optical pickup device which uses the diffraction means to divide the beam emitted from the laser light source into one main beam and two side beams and allows the main photo detector to receive the main beam for reading or writing of the signals and servo error detection, while allowing the individual side photo detectors to receive two side beams for tracking error detection, the method of tracking servo for the optical pickup device according to the present invention comprises the steps of generating the individual push pull signal with the photo detector composed of the main photo detector having four photo-detecting sections crosswise and two side photo detectors having respectively two photo-detecting sections left and right, and then generating the tracking error signal on the basis of the operational output from the push pull signals resulting from canceling the amounts of DC offset produced in the push pull signals in the respective photo detectors every push pull signal.
Thus, the method of tracking servo for the optical pickup device according to the present invention permits the amounts of DC offset produced in the push pull signals in the respective photo detectors to be canceled for each push pull signal. Thus, even though the SPP1 signal and the SPP2 signal are different in amount of DC offset, the DPP signal may be prevented from producing the offset without the need for the operation expression, permitting highly accurate tracking servo simultaneously with attainment of higher recording density.